Cadence refers to the tracking of fields that are used to convert film, which is typically 24 frames per second, to video, which is typically 60 frames per second. Each frame of the film is first converted into two fields, one consisting of even lines and the other consisting of odd lines. The 48 fields are then mapped to video using a predetermined sequence, such as A0, A1, B0, B1, B0, C0, C1, D0, D1, D0, where “A” through “D” represent the original frames of the film and “0” and “1” represent the different field types. The film data is then combined by a video processor to generate video. This sequence is known as 3:2 pull-down, and other sequences can alternatively be used.
Simple cadence following is easy to implement. A counter is incremented at the end of every field, and it is reset to zero after the total number of fields in the sequence have passed. In the case of 3:2 pull-down, the counter counts from zero to four, and is reset to zero. Because the counter free-runs, there is no need to manually reset it. The counter selects a bit from a bitmap that determines whether the current field is merged with the future field or the past field.
The problem with simple cadence following is that the artifacts associated with broken cadences are egregious, where the artifacts associated with dropping from film-mode are not very apparent. As such, a simple cadence follower often results in poor video quality.